The present invention relates to a polymer network containing ion conductive liquids for use as an electrolyte in solid state electrochemical cells.
Presently, solid state electrochemical cells are the subject of intense investigation and development. These cells are described extensively in the patent literature. See, for example, U.S. Pat. Nos. 4,303,748 to Armand et al (1981); 4,589,197 to North (1986); and 4,547,440 to Hooper et al (1985). These cells are characterized by an electrolyte which is a solid solution of an ionic compound dissolved within a plastic macromolecular material.
One problem which research efforts have attempted to overcome in the design of solid state cells is the poor conductivity of polymeric electrolytes at room temperature and below. In many cases, the cells which have been designed to date are used at elevated temperatures due to the low conductivity of the electrolyte at ambient temperatures.
A polymeric electrolyte should provide a combination of both high ionic conductivity and good mechanical strength. Unfortunately, there is a tendency for these two properties to oppose one another. Attempts to increase conductivity usually involve a reduction in molecular weight and result in fluid or mechanically weak films. Techniques, such as crosslinking, increase film strength but generally result in a loss in conductivity.
One effort to improve the electrolyte for a solid state cell is described in Bauer et al, U.S. Pat. No. 4,654,279. This patent discloses a solid polymeric electrolyte which is a two phase interpenetrated network of a mechanically supporting phase of a continuous network of a crosslinked polymer and an ionic conducting phase comprising a metal salt of a complexing liquid polymer such as liquid polyethylene oxide complexed with a lithium salt. As explained in the Bauer et al patent, the mechanically supporting phase forms a matrix which supports an interpenetrating ionically conducting liquid polymer phase which provides continuous paths of high conductivity throughout the matrix. Representative examples of the mechanically supporting phase described in the Bauer et al patent are epoxies, polyurethanes, polyacrylates, polymethacrylates, polystyrenes and polyacrylonitriles.